Fiber-optic communication networks are experiencing rapidly increasing growth in capacity. This capacity growth in fiber-optic networks is generally addressed through advanced modulation formats, higher channel data rates, and/or decreased frequency spacing. Capacity growth is placing a substantial burden on communication systems, primarily in the following two areas. First, increasing optical channel data rates suffers from increasing penalties due to linear impairments, such as amplified stimulated emission (ASE) noise, group velocity dispersion (GVD) and polarization mode dispersion (PMD). ASE is an intrinsic property of the optical amplifier chain, and can be controlled by amplifier design and spacing. GVD is a largely static effect, and has a variety of excellent compensation strategies, including dispersion compensating fiber or gratings, dynamically tunable optical etalons, etc. However, PMD is a rapidly varying dynamic effect and is quite difficult to mitigate. Second, tight frequency spacing of high data rate channels leads to increasing nonlinear cross-talk effects. In particular, cross-phase modulation (XPM) is the more dominant impairment in dense wavelength division multiplexed (DWDM) systems on most conventional fiber types.
Conventional approaches to PMD compensation have generally focused on several areas, such as, (1) Optical PMD compensators based on fast optical polarization tracking, with subsequent optical relative delay (C. Xie, et al, “Automatic optical PMD compensator for 40-Gb/s DBPSK and DQPSK with fast changing SOP and PSP,” ECOC 2008 Proceedings, paper We.3.E.5); (2) Optical PMD compensators based on integrated planar Lightwave circuits to implement the optical filtering required to approximate a response inverse to one generated by fiber PMD (M. Secondini, “Optical equalization: system modeling and performance evaluation,” J. Lightwave Techn., vol. 24, no. 11, November 2006, pp. 4013-4021); (3) Direct-detection receivers, with subsequent compensation based on electronic feed-forward equalizer (FFE) and decision-feedback equalizer (DFE) structures or maximum likelihood sequence estimator (MLSE) estimation (C. Xie, et al, “Performance evaluation of electronic equalizers for dynamic PMD compensation in systems with FEC,” OFC 2007 Proceedings, paper OTuA7); (4) Direct-detection receivers with polarization diversity based on selection of best polarization signal (A. O. Lima, et al, “A novel polarization diversity receiver for PMD mitigation,” IEEE Photon. Techn. Lett., vol. 14, no. 4, April 2002, pp. 465-467); (5) Coherent receivers that allow full E-field capture, with subsequent digital processing of acquired data to remove PMD effects (E. Ip, et al, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Techn., vol. 25, no. 8, August 2007, pp. 2033-2043); and differential receivers with E-field reconstruction capability that also utilize subsequent digital processing to remove PMD effects (X. Liu, “DSP-enhanced differential direct-detection for DQPSK and m-ary DPSK,” ECOC 2007 proceedings, paper 7.2.1).
Optical PMD compensators based on fast optical polarization tracking, with subsequent optical relative delay have several disadvantages. For example, devices used for fast polarization tracking are based on large and expensive lithium niobate (LiNbO3) polarization transformers (i.e., approximately 1 cm×10 cm long brass packaged devices). LiNbO3 polarization transformers require multi-stage control (e.g., five to eight stages) with very high voltages (i.e., in 50-70 V) range, and nanosecond speeds, thereby producing very high associated driver cost and power consumption. Also, differential delays are generally not tunable, and thus two or more stages of PMD compensation may be required to achieve both high range and good precision at the same time. Optical PMD compensators based on integrated planar Lightwave circuits solves the size and cost constraint, but PLC circuit filters are generally thermally tunable. Thus, their response speed is not sufficient to deal with the fast PMD changes observed in fielded systems.
Coherent receivers that allow full E-field capture are excellent for dealing with PMD impairments. Their primary drawbacks include requiring an additional optical local oscillator (LO) laser thereby increasing cost, size, and power. Coherent receivers also require an extremely high performance analog-digital converter (ADC) front end required to digitize four channels (I and Q for both polarizations). These converters typically operate at 2×data symbol rate, and need approximately six bit resolution to provide good signal representation. Thus, a 40 Gbps data channel would need ADCs operating at approximately 22 Gsamples/s, which are hard to realize and consume several Watts of power. Finally, digital signal processing has to continuously operate on very high data rate signals. These require massive parallelization in today's complementary metal oxide semiconductor (CMOS) process, and again consume many Watts of power. Further, E-field reconstruction approaches alleviate coherent LO laser needs, but still require complex and power-hungry ADCs and DSPs, similar to the Coherent receiver approach.